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Think it was Hardball Times last year, or Dave Allen, who demonstrated that Aardsma's fastball looks faster to hitters because of his release point and the locations he tends to use (typically up in the zone where it's hard to get on top of the ball).
We would add that the funky, short-arm, deceptive delivery also increases the apparent speed.
When David Aardsma throws a 94 heater up in the zone, it is as hard to see, apparently, as somebody else's 97-98 fastball.
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Very hard to catch up to a letter-high 94 fastball, but of course the risk is that if it gets squared up, it's going to the warning track at least...